1. Field of the Invention
The present invention relates to an air-to-fuel ratio feedback control system for an internal combustion engine, wherein the air-to-fuel ratio of mixture represented by the concentration of oxygen in the exhaust gases of the engine is detected in the exhaust system and is fed back to the intake system to thereby control the air-to-fuel ratio of mixture supplied to the engine.
2. Description of the Prior Art
In the past, a control system of the above type has been proposed in which in order to clean up the exhaust gases of an internal combustion engine, the concentration of oxygen in the exhaust gases is detected and fed back in such a manner that the air-to-fuel ratio of a mixture supplied to the engine is corrected to a fixed value, e.g., the stoichiometric air-to-fuel ratio. Control by this type of feedback system is particularly advantageous in that the air-to-fuel ratio of the mixture can be automatically controlled at a predetermined value against large variations in external conditions, e.g., the atmospheric pressure, intake air temperature, etc. It has also been confirmed that feedback corrections by integrating the detected value of the air-to-fuel ratio of a mixture is advantageous in increasing the control speed. However, a disadvantage of this system is that if such a feedback control is effected throughout the whole range of operating conditions of the engine, a situation arises in which even during acceleration, full load and other operations where the engine is required to produce a high output power, the mixture is not increased, that is, the air-to-fuel ratio of the mixture is undesirably maintained constant by the feedback control, thus failing to ensure sufficient output power. When the supply of fuel is cut off during the period of deceleration, the prior art feedback control system causes an undesirable situation wherein the mixture is excessively enriched immediately after the termination of the cut-off of the fuel supply this adversely affecting the cleaning of the exhaust gases.